Ethanol is rapidly becoming a major hydrogen-rich liquid transport fuel around the world. Worldwide consumption of ethanol in 2005 was an estimated 12.2 billion gallons. The global market for the fuel ethanol industry has also been predicted to grow sharply in future, due to an increased interest in ethanol in Europe, Japan, the USA, and several developing nations.
For example, in the USA, ethanol is used to produce E10, a 10% mixture of ethanol in gasoline. In E10 blends the ethanol component acts as an oxygenating agent, improving the efficiency of combustion and reducing the production of air pollutants. In Brazil, ethanol satisfies approximately 30% of the transport fuel demand, as both an oxygenating agent blended in gasoline, and as a pure fuel in its own right. Also, in Europe, environmental concerns surrounding the consequences of Green House Gas (GHG) emissions have been the stimulus for the European Union (EU) to set member nations a mandated target for the consumption of sustainable transport fuels such as biomass derived ethanol.
The vast majority of fuel ethanol is produced via traditional yeast-based fermentation processes that use crop derived carbohydrates, such as sucrose extracted from sugarcane or starch extracted from grain crops, as the main carbon source. However, the cost of these carbohydrate feed stocks is influenced by their value as human food or animal feed, while the cultivation of starch or sucrose-producing crops for ethanol production is not economically sustainable in all geographies. Therefore, it is of interest to develop technologies to convert lower cost and/or more abundant carbon resources into fuel ethanol.
CO is a major, free, energy-rich by-product of the incomplete combustion of organic materials such as coal or oil and oil derived products. For example, the steel industry in Australia is reported to produce and release into the atmosphere over 500,000 tonnes of CO annually.
Catalytic processes may be used to convert gases consisting primarily of CO and/or CO and hydrogen (H2) into a variety of fuels and chemicals. Micro-organisms may also be used to convert these gases into fuels and chemicals. These biological processes, although generally slower than chemical reactions, have several advantages over catalytic processes, including higher specificity, higher yields, lower energy costs and greater resistance to poisoning.
The ability of micro-organisms to grow on CO as a sole carbon source was first discovered in 1903. This was later determined to be a property of organisms that use the acetyl coenzyme A (acetyl CoA) biochemical pathway of autotrophic growth (also known as the Woods-Ljungdahl pathway and the carbon monoxide dehydrogenase/acetyl CoA synthase (CODH/ACS) pathway). A large number of anaerobic organisms including carboxydotrophic, photosynthetic, methanogenic and acetogenic organisms have been shown to metabolize CO to various end products, namely CO2, H2, methane, n-butanol, acetate and ethanol. While using CO as the sole carbon source, all such organisms produce at least two of these end products.
Anaerobic bacteria, such as those from the genus Clostridium, have been demonstrated to produce ethanol from CO, CO2 and H2 via the acetyl CoA biochemical pathway. For example, various strains of Clostridium Ijungdahlii that produce ethanol from gases are described in WO 00/68407, EP 1117309, U.S. Pat. Nos. 5,173,429; 5,593,886; and 6,368,819, WO 98/00558, and WO 02/08438. The bacterium Clostridium autoethanogenum sp is also known to produce ethanol from gases (Abrini et al., ARCHIVES OF MICROBIOLOGY 161, pp 345-351 (1994)).
However, ethanol production by micro-organisms by fermentation of gases is always associated with co-production of acetate and/or acetic acid. As some of the available carbon is converted into acetate/acetic acid rather than ethanol, the efficiency of production of ethanol using such fermentation processes may be less than desirable. Also, unless the acetate/acetic acid by-product can be used for some other purpose, it may pose a waste disposal problem. Acetate/acetic acid is converted to methane by micro-organisms and therefore has the potential to contribute to GHG emissions.
Microbial fermentation of CO in the presence of H2 can lead to substantially complete carbon transfer into an alcohol. However, in the absence of sufficient H2, some of the CO is converted into alcohol, while a significant portion is converted to CO2 as shown in the following equations:6CO+3H2O→C2H5OH+4CO2 12H2+4CO2→2C2H5OH+6H2OThe production of CO2 represents inefficiency in overall carbon capture and if released, also has the potential to contribute to Green House Gas emissions.
WO2007/117157, the disclosure of which is incorporated herein by reference, describes a process that produces alcohols, particularly ethanol, by anaerobic fermentation of gases containing carbon monoxide. Acetate produced as a by-product of the fermentation process is converted into hydrogen gas and carbon dioxide gas, either or both of which may be used in the anaerobic fermentation process. WO2008/115080, the disclosure of which is incorporated herein by reference, describes a process for the production of alcohol(s) in multiple fermentation stages. By-products produced as a result of anaerobic fermentation of gas(es) in a first bioreactor can be used to produce products in a second bioreactor. Furthermore, by-products of the second fermentation stage can be recycled to the first bioreactor to produce products.
U.S. Pat. No. 7,078,201 and WO 02/08438 also describe improving fermentation processes for producing ethanol by varying conditions (e.g., pH and redox potential) of the liquid nutrient medium in which the fermentation is performed. As disclosed in those publications, similar processes may be used to produce other alcohols, such as butanol.
Fermentation of gaseous substrates can be challenging due to the requirement that at least a portion of the gaseous substrate is dissolved in a typically aqueous fermentation broth before the substrate can be metabolised by a microbial culture. Fermentations involving gaseous substrates, wherein one or more gaseous components are the carbon and optionally the energy source for a microorganism, are particularly challenging due to the large amount of substrate required to be solubilised in a fermentation broth before any metabolism can take place. Examples of gaseous substrates used as a carbon and/or energy source in fermentation include CO, CO2, CH4, H2 and H2S. In particular, sparingly soluble substrates, such as CO and/or H2 require highly efficient mass transfer into an aqueous fermentation broth as CO is both a carbon and energy source for anaerobic fermentation. For example, the theoretical equations for CO and H2 to ethanol are:6CO+12H2→3C2H5OH+3H2OThus, six molecules of gas (CO and/or H2) must be dissolved in a fermentation broth to produce one molecule of ethanol.
Mass transfer of a gas into a liquid is a function of three main variables:                1. Concentration Driving Force: The partial pressure of a particular gaseous component is substantially proportional to the rate at which that component can be driven into a solution.        2. Interfacial Surface Area: The larger the interfacial surface area between gas and liquid phases, the higher the opportunity for mass transfer. In particular, the interfacial surface area is typically a function of gas hold-up and bubble size.        3. Transfer Coefficient: The transfer coefficient of a system is influenced by a variety of factors. However, from a practical perspective, typically the largest influence is the relative velocity between the liquid and the gas phases. Relative velocities (and hence mass transfer) are typically increased by increasing turbulence through agitation or other mixing.        
Various devices for the modification of fluid flow, e.g., to enhance mixing and/or otherwise improve control of multi-phase (gas-liquid) processes are described in U.S. Pat. No. 6,333,019; U.S. Pat. No. 6,742,924; WO 2009/124939; and WO 2012/042245. In the case of processes for fermentation of gaseous substrates the efficient mass transfer of the gas into solution is only one of several variables affecting the degree of gas utilization or consumption by the bacterial culture. For example, once the gaseous feedstock is dissolved, it must be retained in the culture for a sufficient time for transfer to, and consumption by, the microbes. Both the energy input for circulating the liquid culture medium (e.g., in a loop from an upper section of a riser, back to a lower section near the gaseous feed inlet) in combination with the particular choice of equipment, are important considerations affecting CO utilization in two-phase biological fermentation processes, and consequently their overall economic viability. In this regard, mixing or gas-liquid contacting devices used conventionally to enhance gas-liquid mass transfer typically require large amounts of energy in order to attain the desired results in terms of overall CO utilization. To achieve effective fermentation of CO and optionally H2 into products, such as acid(s) and alcohol(s), the CO-containing substrate must be not only dissolved, but also made available to the microorganisms in an efficient manner. Mechanical means, such as vigorous stirring, used conventionally for mass transfer improvement alone, often do not provide acceptable process economics in biological fermentation processes, as they require a large power input, which becomes inefficient and/or uneconomical as scale increases.
Even minor improvements to a fermentation process for producing one or more acids and/or one or more alcohols can have a significant impact on the efficiency, and more particularly, the commercial viability, of such a process. The present invention relates to system(s) and/or method(s) that overcome disadvantages known in the art and provide novel solutions for the optimal production of a variety of useful products.